This research will assess alterations in the state of ion selective channels in membranes of lobster axons induced by sensitized photochemical modification, using the double sucrose gap voltage clamp method. The rate constants for change in excitability parameters will be used as an assay. Experiments will attempt to define the nature of the site where modification occurs. One set of experiments will be aimed at determining the parallels between hydrogen ion modification and photochemical modification of channels. Another set will be aimed at studying the interaction between group-specific protein reagents and photochemical modification.